In communications systems, multiplexing is a commonly used process for transmitting multiple analog message signals or digital data streams over a shared communications medium. In general, the multiplexing process divides the capacity of a low-level communications channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. The two most basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM), both either in analog or digital form.
To supplement multiplexing techniques, orthogonal multiplex schemes are commonly used. That is, a large number of closely spaced orthogonal sub-carriers can be used to carry data as a collection of parallel data streams. In an orthogonal scheme, an ideal receiver can completely reject arbitrarily strong unwanted signals using different orthogonal basis functions or waveforms than included in the received signal. One exemplary orthogonal transmission scheme is TDMA, where the orthogonal basis functions are non-overlapping rectangular pulses. Another exemplary scheme is orthogonal frequency-division multiplexing (OFDM) is commonly utilized as a digital multi-carrier modulation method.
The orthogonality of the sub-carriers in such schemes typically results in high spectral efficiency, with a total symbol rate near the Nyquist rate. Accordingly, almost the whole available frequency band can be utilized for transmitting symbols. Furthermore, the orthogonality also effectively results in a simplification of channel equalization efforts since the operation of a multiplexed orthogonal scheme effectively results in converting a rapidly modulated wideband signal into a collection of slowly modulated narrowband signals. Consequently, the typical low symbol rate in each sub-carrier makes the use of guard intervals between symbols practical, making it possible to more effectively address time-spreading issues and reduce or eliminate inter-symbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.